This invention relates in general to photovoltaic devices; and their preparation, and more particularly, to combinations including thin film organic polymer semiconductors which, in conjunctions with appropriate electrodes, function to convert light into electricity.
It is well known that a part of the energy of sunlight can be captured and converted directly to electricity by photovoltaic devices otherwise known as solar cells. These devices are generally fabricated from inorganic materials such as silicon, gallium arsenide, cadmium sulfide, etc. The mechanism for this type of solar energy conversion, known as the photovoltaic effect, has been described in the literature of solid state physics (W. D. Johnston, Jr. American Scientist, Nov/Dec 1977, p. 729). In certain types of materials, known as semiconductors, a photon which is an elementary unit of light energy, may transfer part of its energy to raise a negative charge carrier (electron) from a filled state (valence band) to an empty state (conduction band) leaving behind the absence of an electron which behaves like a positive charge carrier (hole). Solar cells have an internal (solid state) characteristic which promotes the separation of such photogenerated carriers (electrons and holes); and the resultant potential gradient initiates a flow of electricity in an attached external circuit. In practical prior art situations it has been possible to convert as much as 20% of the energy of the sunlight at air mass zero into electrical energy for either immediate use or storage.
Although the materials used in some solar cells are plentiful on the earth's crust, they are expensive to isolate in a form pure enough and of such quality as to be free of impurities and/or defects. Silicon, for example, must be freed of impurities down to the parts-per-billion level and then, for some applications, carefully grown into single crystals. The material must be fabricated to have a p-n junction or Schottky barrier which promotes the separation of the photogenerated carriers. The foregoing processes are expensive and not easily adaptable to mass production.
According to well-known solid state theory, the voltage which can be developed by a solar cell is a function of the excess of minority carriers on either side of the potential gradient junction. Minority carriers take the form of holes in n-type material and electrons in p-type material. The voltage so developed is less than the band gap (minimum energy required to create an electron-hole pair because of junction losses. For example, solar cells fabricated from silicon (band gap=1.1 electron volts) typically have an open-circuited voltage of about 0.5 volts. Similar magnitudes of open-circuit voltages have been measured for organic semiconductor photocells (A. K. Ghosh and T. J. Teng, J. Appl. Phys., Volume 44, 2781, (1973); Volume 45, 230, (1974).
There are various loss processes which reduce the efficiency of conversion of sunlight to electricity. Out of the spectrum of photon energies which comprise sunlight, only those with energy greater than the band gap (for example, 1.1 electron volts for silicon) can be absorbed to produce photocharges. This amounts to 77% of the incoming solar light in the case of silicon. Part of the light is reflected off the surface of the crystal because of the differing refractive indices of the material. For example, the refractive index:
n.sub.s (for silicon)=3.4; and PA1 n.sub.a (for air)=1 Then, reflectivity: ##EQU1## PA1 F.sub.A =1-e.sup.-.alpha.t
Thus, approximately 30% of the photons in sunlight are of the proper energy, are not reflected, and can enter the crystal to be absorbed.
(2) The fraction absorbed in the crystal:
Where .alpha.=the absorption constant; PA2 t=the thickness of the crystal, and PA2 e.sup.-.alpha.t =the fraction of light transmitted through the photovoltaic cell.
The optimal depth of the p-n junction region beneath the face of the crystal is dictated by such factors as where in the crystal most of the light is absorbed so as to produce electron and hole pairs, the lifetimes and mobilities of the photocarriers, and the resistance of the very thin side of the junction next to the surface through which the current must flow to reach the front contact electrode. This latter resistance is governed predominantly by the geometry of the photovoltaic cell, while the other phenomena are functions of the material characteristics of the crystal itself. In general, high quality, low resistivity material is needed to enhance the lifetime of the photocarriers, prevent their recombination (a loss process), and allow them to diffuse further. This increases their chances of encountering the electric field at the junction, being separated, and producing a current.
In the prior art, the principal difficulty with the efficient conversion of light energy into electrical energy by organic materials has been their high resistivity. Organic materials have no scarcity of carriers; but in general, they are feeble conductors of electricity even under bright illumination.